1. Field of the Invention
This invention related generally to wireless communication systems. More specifically, the invention relates to digital demodulation of quadrature phase shift keying (QPSK) signals.
2. Description of the Related Art
The demodulation of QPSK signals can be accomplished digitally employing a tracking loop approach. A tracking loop generates the resample timing and is also used to remove the frequency and phase offsets, also referred to as the residual frequency and phase, from the incoming data symbols. Typically, such systems operate on one new sample pair at a time. Such systems employ feedback loops to track and correct for timing, frequency and phase offsets in the incoming data stream.
These feedback loops require careful setting of the tracking loop gains. In addition, they require an acquisition period over which the feedback loops have time to lock. Such known systems require the use of a relatively long preamble at the beginning of each new data transmission in order to provide the feedback loops time to lock. Such known systems also typically estimate timing, frequency and phase using localized loop metrics (typically over a few data samples), which can lead to increased error rates. Further, due to the local decisions used in tracking loops, implementing such systems in a digital signal processor chip limits the processing speeds with which such systems can operate.
Therefore, there is a need for method and apparatus for digitally demodulating QPSK signals, which overcomes these shortcomings.